1. Field of the Invention
The present invention relates to an optical receiver and a method to control the optical receiver.
2. Related Prior Arts
An electrically compensating method for the dispersion attributed to a multimode fiber has been developed. The method is called as the electrical dispersion compensator (hereafter denoted as EDC). A Japanese patent application published as JP-H08-163027A has disclosed one of circuits applicable to the EDC. The circuit disclosed therein is a type of optical signal processor and includes a plurality of delay units that sequentially delays a signal converted from the received optical signal and outputs thus delayed signals in parallel, a plurality of multipliers each coupled with one of delay units and multiplying the delayed signal by a coefficient unique to the delay unit, an adder that sums up respective outputs of the multiplier, and an arithmetic unit to evaluate the coefficients, which is one type of, what is called, transversal filter.
Optimum multiplied coefficients set in the multipliers strongly depend on the optical transmission line. The aged deterioration, the tensile stress or the bent status of the fiber, and so on, affects the optimum multiplied coefficients. Thus, the EDC is necessary to evaluate the optimum coefficients by the arithmetic unit. However, depending on the conditions of the transmission line, the arithmetic unit is occasionally unable to evaluate the optimum multiplied coefficients, which results in a drastic increase of the bit error rate.
Even in such a case that the arithmetic unit could not reach the optimum coefficients, the transversal filter sometimes gives the optimum set of multiplied coefficients by resetting whole coefficients. However, the resetting the filter means that, the optical receiver implementing this filter is unavoidable to be temporarily ceased. Thus, an aspect of the present invention is to provide an optical receiver with a function to adjust or to reset the multiplied coefficients of the transversal filter without suspending the optical receiver.